Pumping stations for water supply from river intakes or boreholes
are normally designed to discharge continuously over a period of 20–22 h a
day. Boosters may also need storage facilities.
It is advisable to install more than one pump so that maintenance
may be carried out without interrupting the supply. Often, several pumps
of differing capacities are installed in a water supply pumping station, and
the units are switched on or off as demand variations dictate. If a very fine
controlled output is needed, variable-speed motors (expensive) must be
used.
Sewage pumps must be so designed as to be capable of pumping the
peak flows as they occur in sewers, with a maximum delay of no more than
12 h. Otherwise, the sewage may become septic. The flows may vary con-
siderably throughout the year, month, week or day. The daily variations
may be considered to be between zero (at night) to about six times the dry
weather flow (d.w.f.) at times during the day, and it is preferable to install
pumping capacity up to 6 d.w.f. using at least two or three pumps, each
capable of about 2 d.w.f.
Pumping mains are usually designed for velocities of around 0.9–
1ms^1 when supplying water at a constant rate throughout the day. This
may be doubled for short-period pumping. Prior to the final selection of
the pipeline diameter, economical analyses balancing the costs of large-
diameter pipelines and their maintenance, and the savings in pumping
costs due to the reduction in friction losses, must be carried out.
The minimum diameter suitable for pumping unscreened crude
sewage is 100 mm and velocities around 0.75–2 m s^1 are normally recom-
mended. However, in stormwater–sewage pumping higher velocities (even
3–4 m s^1 ) may be more economical as their operation is intermittent. The
friction head losses in the case of sludge flows are much larger than for
clear water flows.
The suggested (Bartlett, 1978) multiplying factors, F, for the calcula-
tion of head losses in sludge flows, given by
hfF(L/D)V^2 /2g (13.24)(whereis the friction factor for clear water flows), are given in Table
13.3.
A pumping station normally consists of a substructure below ground
level in two compartments (if a dry well sump is used; Fig. 13.8). The roof
of the dry well forms the floor of the superstructure of the pumping
station, with appropriate access to the pumping units. The access to the
wet well (sump) is normally provided from outside. The functional
requirements, with the necessary dimensions, must be carefully assessed
and incorporated in the design.